"Dumb phone" is a coarse reference to the largely obsolete portable cellular telephones (of differing brands) made from 1983 to 1999. Majority of them were manufactured by Motorola from 1983 to 1999. Other manufacturers during that period include Ericsson, Sony, Nokia, Mobira and Oki.

Contrary to popular belief, dumb phones do not refer to feature phones (mobile phones made from 2000 - present with little features as compared to smart phones).

Al: Did you see that New Zealand ad? The one about the gal who used a dumb phone 'coz her insurance plan was so damn expensive?

The Urban Dictionary T-Shirt

Opposite of a smartphone. Generally has lower specs than a smartphone in terms of screen size, wireless connectivity, storage space etc. However the critical differentiator is that there is no development environment that allows any third party applications to be installed (other than java midlets which are very limited). Manufacturers and networks call them 'feature phones'.

This is a near-exhaustive list of smartphone OSes. If a phone doesn't use something on this list it is probably a dumbphone: